It clearly works pretty well that way. Textual (Mac IRC client) is the same setup.

Slack, on the other hand, feels like JavaScript frameworks piled three or four deep, often getting its UI out of sync with the actual state.

Chats/channels will briefly flash (1) unread in the sidebar to tell me about my own message that I just sent, or I’ll continue to have a blue “10 new messages” banner in a conversation that I’m actively participating in.

I use it because it’s convenient, but there’s definitely room for improvement.

Wow, that's crazy. The iOS Messages app is native. I'm only familiar with iOS development... is AppKit that bad?

UIKit is noticeably better than AppKit but I doubt this is the reason why Messages is webview based. I don't know the exact reason behind it, but my guess is that they stopped caring about updating it when the people working on it either left or moved over to other projects.

I've taken that same guess after the new fancy effects and animations for iMessage got fully implemented in iOS 10 and barely supported at all in macOS Messages. It seemed like an afterthought on the Mac.